As is generally known, in an automatic transmission mounted on an automotive vehicle, hydraulic pressure used to engage (apply) or disengage (release) each of friction elements, such as clutches and band brakes, and hydraulic pressure used for lubrication of sliding/moving transmission parts with lubricating oil are generated by means of an oil pump. In the automotive automatic transmission, hydraulic pressure has to be generated and supplied by the oil pump, irrespective of whether the vehicle is running or in its stopped state. Thus, in an automatic transmission using a torque converter, an oil pump is laid out in the rear of a torque-converter pump impeller, which is rotated in synchronism with rotation of an engine crankshaft. An inner rotor of the oil pump is fixedly connected to the pump impeller so as to always drive the oil pump during rotation of the crankshaft.
Such a technology related to a pump structure of an oil pump laid out between a torque converter and a transmission mechanism has been disclosed in Japanese Patent Provisional Publication No. 2-35904 (hereinafter is referred to as “JP2-35904”).
Referring now to FIGS. 7-8, there is shown the oil pump structure disclosed in JP2-35904. As can be seen from the simplified drawing of FIG. 7, an oil pump 201 is laid out between a torque converter 211 and a transmission mechanism 212. A pump body 202 is laid out or installed on one side facing a pump impeller of torque converter 211, whereas a pump cover 203 is laid out or installed on the opposite side facing transmission mechanism 212. Pump body 202 and pump cover 203 are integrally connected to each other to define therein a pumping chamber 204. An inner rotor and an outer rotor are operably accommodated in pumping chamber 204. A separate plate (not shown) is interleaved between pump body 202 and pump cover 203 so as to ensure a high sealing performance of fluid passages formed in oil pump 201. As shown in FIG. 8, one end of an oil passage formed in pump body 202 communicates with a discharge portion (an outlet port) 222 of an oil strainer 221. Lubricating oil supplied to sliding/moving transmission component parts in transmission mechanism 212 is sucked through a suction portion (an inlet port) 223 of strainer 221. The sucked oil is filtered out through an oil filter 224 of strainer 221. The filtered-out oil flows through discharge portion 222 of strainer 221 via a suction port 202a into pump body 202, that is, into the space defined between the inner and outer rotors for pumping action. On the other hand, working oil drained from each of hydraulic servos for friction elements such as clutches and band brakes is exhausted from a valve body (exactly, a control valve body) 231 into an oil passage 214 formed in a transmission case 213 of transmission mechanism 212, and then flows through discharge portion 222 of strainer 221 via suction port 202a into pump body 202.
That is to say, as can be seen from the cross section of FIG. 8, the discharge portion 222 of strainer 221 has a fluid-flow communication opening 222b communicating with the oil passage 214 of transmission case 213 and a fluid-flow communication opening 222a communicating with the suction port 202a of pump body 202. Thus, oil, which is used as lubricating oil for transmission mechanism 212 and discharged from the discharge portion 222 of strainer 221, and oil, which is drained from each of hydraulic servos and directed to pump body 202, are merged with each other at the discharge portion 222 of strainer 221 (that is, into the space defined between the above-mentioned two fluid-flow communication openings 222a-222b). The merged oil flows through suction port 202a into the pumping chamber of pump body 202.
To provide leakproof oil seals and to prevent oil leakage, a pair of O-ring seals 225, 225 are respectively interleaved between the opening end of oil passage 214 of transmission case 213 and the first fluid-flow communication opening 222b of discharge portion 222 of strainer 221 and between the second fluid-flow communication opening 222a of discharge portion 222 of strainer 221 and the suction port 202a of pump body 202.
Additionally, in the oil pump structure disclosed in JP2-35904, as shown in FIG. 7, generally, the diameter of the outer circumference of pump body 202 is dimensioned to be greater than that of pump cover 203. The oil inflow port, i.e., suction port 202a (see FIG. 8) is formed in the outer peripheral portion of pump body 202, further projected radially from the outer circumference of pump cover 203. This is because the subjective component part of oil pump 201 is the pump body 202, which also serves as an end cover of transmission mechanism 212. That is, oil pump 201 is constructed by integrally connecting pump body 202 onto the end face of transmission mechanism 212, and therefore major structural portions of oil pump 201, for example, pumping chamber 204 and suction port 202a tend to be all provided in pump body 202 rather than pump cover 203.